Fuel cell

ABSTRACT

A fuel cell includes an anode, a solid electrolyte layer, a barrier layer, and a cathode. The anode includes a transition metal and an oxygen ion conductive material. In the interface region within 3 micrometers from the interface with the solid electrolyte layer of the anode after reduction, the content rate of silicon is less than or equal to 200 ppm, the content rate of phosphorous is less than or equal to 50 ppm, the content rate of chromium is less than or equal to 100 ppm, the content rate of boron is less than or equal to 100 ppm, and the content rate of sulfur is less than or equal to 100 ppm.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation of application Ser. No. 13/870,442,filed Apr. 25, 2013, and entitled “Fuel Cell”, which is a continuationof PCT/JP2012/075965, filed Oct. 5, 2012, which claims priority to JP2011-27718, filed Oct. 14, 2011, JP 2012-136509, filed Jun. 18, 2012, JP2012-196183, filed Sep. 6, 2012, and JP 2012-214868, filed Sep. 27,2012, the entire contents of each of which are hereby incorporated byreference.

TECHNICAL FIELD

The present invention relates to a solid-oxide fuel cell.

BACKGROUND ART

A solid-oxide fuel cell generally includes an anode, a cathode and asolid electrolyte layer that is disposed between the anode and thecathode. The anode generally contains a transition metal such as nickelor the like and an oxygen ion conductive material such asyttria-stabilized zirconia or the like (for example, reference is madeto Japanese Patent Application Laid-Open No. 2006-32132).

SUMMARY Technical Problem

However, since the output of the fuel cell disclosed in Japanese PatentApplication Laid-Open No. 2006-32132 exhibits a tendency to decrease,there is a need for an improvement in relation to maintenance of theoutput of the fuel cell.

The present invention is conceived in light of the above situation, andit is an object thereof to provide a fuel cell in which a decrease inoutput can be suppressed.

Solution to Problem

A fuel cell according to a first aspect of the present inventioncomprises an anode, a cathode, and a solid electrolyte layer that isdisposed between the anode and the cathode. The anode after reductionhas an interface region within 3 micrometers from the interface betweenthe solid electrolyte layer and the anode. In the interface region, acontent rate of silicon is less than or equal to 200 ppm, a content rateof phosphorus is less than or equal to 50 ppm, a content rate of chromeis less than or equal to 100 ppm, a content rate of boron is less thanor equal to 100 ppm, and a content rate of sulfur is less than or equalto 100 ppm.

A fuel cell according to a second aspect of the present inventioncomprises an anode, a cathode, and a solid electrolyte layer that isdisposed between the anode and the cathode. The anode includes nickeland an oxygen ion conductive material. In a cross section of the anode,the anode has a region in which an average value of a contact length ofa particle of nickel and a particle of the oxygen ion conductivematerial is greater than or equal to 0.4 micrometers and less than orequal to 0.9 micrometers.

Advantageous Effects

The present invention provides a fuel cell in which a decrease in outputcan be suppressed.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a sectional view illustrating the configuration of a fuelcell.

FIG. 2 is a SEM image of the cross section of an anode active layer.

FIG. 3 illustrates the analysis results of the SEM image illustrated inFIG. 2.

FIG. 4 is a distribution graph illustrating the distribution of thecontact length between a Ni particle and 8YSZ particle contactinterface.

FIG. 5 illustrates the analysis results of connected Ni particles andisolated Ni particles (FE-SEM using an in-lens secondary electrondetector).

DESCRIPTION OF EMBODIMENTS

Next, the embodiments of the present invention will be described makingreference to the figures. In the description of the figures below, thesame or similar portions are denoted by the same or similar referencenumerals. However, the figures are merely illustrative and the ratio ofrespective dimensions or the like may differ from the actual dimensions.Therefore, the actual dimensions or the like should be determined byreference to the following description. Furthermore, it goes withoutsaying that the ratios or the relations of dimensions used in respectivefigures may be different.

In the following embodiments, a solid oxide fuel cell (SOFC) will bedescribed as an example of a fuel cell battery. In the followingdescription, although a so-called flat-tubular type fuel cell battery isdescribed, the invention is not limited in this regard, and may beapplied to a so-called segmented-in-series fuel cell battery

Configuration of Fuel Cell 10

The configuration of a fuel cell (abbreviated below to “cell”) 10 willbe described making reference to the figures. FIG. 1 is a sectional viewof the configuration of the cell 10.

The cell 10 is a thin tabular body composed of a ceramic material. Thethickness of the cell 10 may be for example 300 micrometers to 3 mm, andthe diameter of the cell 10 may be 5 mm to 50 mm. A plurality of cells10 is connected in series by an interconnector to form a fuel cellbattery.

The cell 10 includes an anode 11, a solid electrolyte layer 12, abarrier layer 13 and a cathode 14.

The anode 11 functions as an anode of the cell 10. The anode 11 asillustrated in FIG. 1 is configured from an anode current collectinglayer 111 and an anode active layer 112. In the present embodiment,although the anode 11 is assumed to be in a reduced state, the contentrate (ppm) of each composition that configures the anode 11 isapproximately fixed before and after reduction.

The anode current collecting layer 111 is a porous tabular fired bodyincluding a transition metal and an oxygen ion conductive material. Theanode current collecting layer 111 includes nickel oxide (NiO) and/ornickel (Ni) as the transition metal. The oxygen ion conductive materialin the anode current collecting layer 111 may include a zirconia-basedmaterial such as scandia-stabilized zirconia (ScSZ) andyttria-stabilized zirconia (8YSZ, 10YSZ, or the like), or a ceria-basedmaterial such as gadolinium doped ceria (GDC (Ce,Gd)O₂) and samariumdoped ceria (SDC ((Ce, Sm)O₂), or yttria (Y₂O₃).

The thickness of the anode current collecting layer 111 may be 0.2 mm to5.0 mm. The thickness of the anode current collecting layer 111 may bethe largest of each constituent member of the cell 10 when functioningas a base plate. The volume ratio of Ni and/or NiO in the anode currentcollecting layer 111 may be 35 to 65 volume % using an Ni conversion,and the volume ratio of the oxygen ion conductive material may be 35 to65 volume %.

The anode active layer 112 is disposed between the anode currentcollecting layer 111 and the solid electrolyte layer 12. The anodeactive layer 112 is a porous tabular fired body including a transitionmetal and an oxygen ion conductive material. The anode active layer 112includes at least NiO and/or Ni as the transition metal. The transitionmetal used in the anode active layer 112 may further include Fe₂O₃ (orFeO) and/or CuO and/or Cu. The oxygen ion conductive material in theanode active layer 112 may include a zirconia-based material such asyttria-stabilized zirconia (8YSZ, 10YSZ, or the like) andscandia-stabilized zirconia (ScSZ), or a ceria-based material such asgadolinium doped ceria (GDC (Ce,Gd)O₂) and samarium doped ceria (SDC((Ce, Sm)O₂).

The thickness of the anode active layer 112 may be 1.0 micrometers to30.0 micrometers. The volume ratio of Ni and/or NiO in the anode activelayer 112 may be 25 to 50 volume % using an Ni conversion, and thevolume ratio of the oxygen ion conductive material may be 50 to 75volume %. In this manner, the content ratio of the oxygen ion conductivematerial in the anode active layer 112 may be greater than in the anodecurrent collecting layer 111.

In a region of the anode 11 that is within 3 micrometers from theinterface Q between the solid electrolyte layer 12 and the anode 11(hereinafter referred to as “interface region R”), the content rate ofsilicon (Si) is less than or equal to 200 ppm, the content rate ofphosphorus (P) is less than or equal to 50 ppm, the content rate ofchrome (Cr) is less than or equal to 100 ppm, the content rate of boron(B) is less than or equal to 100 ppm, and the content rate of sulfur (S)is less than or equal to 100 ppm. In particular, in the interface regionR of the anode 11, it is preferred that the content rate of Si is lessthan or equal to 100 ppm, the content rate of P is less than or equal to30 ppm, the content rate of Cr is less than or equal to 50 ppm, thecontent rate of B is less than or equal to 50 ppm, and the content rateof S is less than or equal to 30 ppm. Furthermore, in the interfaceregion R of the anode 11, it is preferred that the content rate of Si,P, Cr, B and S is greater than or equal to 1 ppm. The respective contentrates of Si, P, Cr, B and S in the interface region R for example may bemeasured using secondary ion-microprobe mass spectrometry.

The respective amounts of Si, P, Cr, B and S contained in the reducedanode 11 may be present in an inner portion of the transition metalparticles, or in the particle interface between transition metalparticles.

As illustrated in FIG. 1, although the interface region R according tothe present embodiment is configured as a portion of the anode activelayer 112, there is no limitation in this regard. As long as thethickness of the anode active layer 112 may be less than or equal to 3micrometers, the interface region R may constitute the whole of theanode active layer 112.

It is preferred that the respective content rate of Si, P, Cr, B and Sis controlled to the above values in relation to the entire anode activelayer 112 and not only the interface region R. That is to say, in theentire anode active layer 112, it is preferred that the content rate ofSi is less than or equal to 200 ppm, the content rate of P is less thanor equal to 50 ppm, the content rate of Cr is less than or equal to 100ppm, the content rate of B is less than or equal to 100 ppm, and thecontent rate of S is less than or equal to 100 ppm. It is still morepreferred that in the entire anode active layer 112 the content rate ofSi is less than or equal to 100 ppm, the content rate of P is less thanor equal to 30 ppm, the content rate of Cr is less than or equal to 50ppm, the content rate of B is less than or equal to 50 ppm, and thecontent rate of S is less than or equal to 30 ppm. In the entire anodeactive layer 112, it is still more preferred that the respective contentrate of Si, P, Cr, B and S is greater than or equal to 1 ppm.

The anode active layer 112 has a plurality of pores. The pore ratio inthe anode active layer 112 is preferably greater than or equal to 10%and less than or equal to 40% in a state after known reductionprocessing (for example, a process in which NiO is reduced to Ni in ahydrogen atmosphere at 800 degrees C.). A pore ratio is the surfaceoccupied ratio of all pores that are exposed on the surface relative tothe surface area of the anode active layer 112. However, the pore ratiomay be the volume occupied ratio of all pores relative to the volume ofthe anode active layer 112. The microstructure of the anode active layer112 will be described below.

The solid electrolyte layer 12 is disposed between the anode 11 and thebarrier layer 13. The solid electrolyte layer 12 has the function ofenabling transmission of oxygen ions generated in the cathode 14. Thesolid electrolyte layer 12 includes zirconium (Zr). The solidelectrolyte layer 12 may include Zr in the form of zirconia (ZrO₂). Thesolid electrolyte layer 12 may include ZrO₂ as a main component. Inaddition to ZrO₂, the solid electrolyte layer 12 may include an additivesuch as Y₂O₃ and/or Sc₂O₃. These additives may function as a stabilizer.In the solid electrolyte layer 12, the stabilizer may have a molcomposition ratio with respect to the stabilizer ZrO₂ (stabilizer: ZrO₂)of 3:97-20:80. In other words, the material used in the solidelectrolyte layer 12 may include zirconia-based materials such as ScSZor the like and yttria-stabilized zirconia such as 3YSZ, 8YSZ, and10YSZ, or the like. The thickness of the solid electrolyte layer 12 maybe 3 micrometers to 30 micrometers.

The barrier layer 13 is disposed between the solid electrolyte layer 12and the cathode 14. The barrier layer 13 has the function of suppressingformation of a high resistive layer between the solid electrolyte layer12 and the cathode 14. The material used in the barrier layer 13includes cerium (Ce) and a ceria-based material including a rare earthmetal oxide in solid solution in Ce. More specifically, the ceria-basedmaterial includes GDC, SDC, or the like. The thickness of the barrierlayer 13 may be 3 micrometers to 20 micrometers.

The cathode 14 is disposed on the barrier layer 13. The cathode 14functions as the cathode of the cell 10. The cathode 14 may contain aperovskite-type complex oxide including lanthanum as a main component.The perovskite-type complex oxide including lanthanum for exampleincludes LSCF (lanthanum strontium cobalt ferrite), lanthanum manganite,lanthanum cobaltite, and lanthanum ferrite. The perovskite-type complexoxide containing lanthanum may be doped with strontium, calcium,chromium, cobalt, iron, nickel, aluminum, and the like. The thickness ofthe cathode 14 may be 10 micrometers to 100 micrometers.

Microstructure of Anode Active Layer 112

In the following description, the preferred microstructure of the anodeactive layer 112 will be described making reference to FIG. 2 to FIG. 5in sequence. The description is based on the premise that the anode 11is in a reduced state.

(1) SEM Image

FIG. 2 is a sectional SEM image of the anode active layer 112 enlargedwith a magnification of 3000 times by a field emission scanning electronmicroscope (FE-SEM) using an in-lens secondary electron detector. FIG. 2illustrates a sectional view of the anode active layer 112 that iscomposed of Ni-8YSZ. In the sectional view of the anode active layer112, after polishing with precision machinery, ion milling processing isperformed using an IM4000 manufactured by Hitachi High-TechnologiesCorporation. FIG. 2 is an SEM image obtained by an FE-SEM (model:ULTRA55) manufactured by Zeiss AG (Germany) with a working distancesetting of 2 mm, and an acceleration voltage of 1 kV. The size of onefield of view as illustrated in FIG. 2 has dimensions of vertical 7micrometers×horizontal 34 micrometers.

In the SEM images, the contrast enables individual display of Niparticles, 8YSZ particles, and the pores, and in FIG. 2, the Niparticles are displayed with a “white” color, the 8YSZ particles with a“black” color and the pores with a “grey” color. However, in FIG. 2, aportion of the contour of the pores is displayed in whiteout condition.

The method of discriminating the Ni particles, the 8YSZ particles andthe pores is not limited to use of contrast in a SEM image. For example,after acquisition of element mapping using SEM-EDS in the same field ofview, trivalent values can be obtained for the Ni particles, the 8YSZparticles and the pores by checking against the FE-SEM image (includingthe in-lens image and the out-lens image) previously obtained by use ofthe in-lens secondary electron detector to thereby identify respectiveparticles in the SEM image. The identification of each particle issimplified by determining that low-luminance regions on the out-lensimage are pores, and then determining that high-luminance regions in aregion other than the pores on the in-lens image are Ni particles, andlow-luminance regions are 8YSZ particles.

In the present embodiment, an Ni particle is an example of “a transitionmetal particle” and an 8YSZ particle is an example of an “oxygen ionconductive material particle”.

(2) Analysis of SEM Image

FIG. 3 illustrates the image analysis results for the SEM imageillustrated in FIG. 2 using HALCON image analysis software produced byMVTec GmbH (Germany). In FIG. 3, the contacting line of the Ni particlesand the 8YSZ particles is shown by the solid line, the interfaceboundary of the Ni particles and the pores (hereinafter referred to as“first interface line”) is shown by the broken line, and the interfaceboundary of the 8YSZ particles and the pores (hereinafter referred to as“second interface line”) is shown by the dot-dash line. As illustratedin FIG. 3, in an inner portion of the anode active layer 112, the Niparticles is connected to the 8YSZ particles at a plurality of spots.Furthermore, the Ni particles and the pores are adjacent at a pluralityof spots, and the 8YSZ particles and the pores are adjacent at aplurality of spots.

(3) Distribution Graph of Contact Length

FIG. 4 is a distribution graph of the contact length of the Ni particlesand the 8YSZ particles. The distribution graph is prepared based on theimage analysis results for one field of view as illustrated in FIG. 3.In the present embodiment, data for contacting positions of less than orequal to 0.2 micrometers recognized by the image analysis software isnot used in relation to the calculation of the average value of thecontact length as described below, or in the calculation of the standarddeviation of the average value of the contact length. This is due to thefact that confirmation of the presence of a contacting position at 0.2micrometers or less as recognized by the image analysis software is notpossible according to the results of observation at a highermagnification, and therefore the contacting position of 0.2 micrometersor less has been determined to be not suitable for consideration forinclusion as a factor to regulate the output performance, ordeterioration of the anode.

The average value of the contact length of the Ni particles and the 8YSZparticles (hereinafter referred to as “average value of contact length”)is preferably greater than or equal to 0.4 micrometers and less than orequal to 0.9 micrometers. The average value of the contact length is avalue calculated by dividing the sum of the contact lengths of the Niparticles and the 8YSZ particles by the number of contacting spots, andin the example illustrated in FIG. 4, takes a value of 0.63 micrometers.The average value of the contact length may be calculated using theanalysis results in one field of view or a plurality of fields of view,and the magnification of the SEM image is not limited to 3000 times.

The average value of the contact length is an index that indicates thebond width of the Ni particles and the 8YSZ particles. A bond that has asufficient and requisite width between the Ni particles and the 8YSZparticles can be formed by controlling the average value of the contactlength to a predetermined range. In this manner, since a strongconnected network can be created between the Ni particles and the 8YSZparticles which form the backbone, a variation in the configuration ofthe Ni particles can be suppressed, and it is possible to preserve thereaction field in the anode active layer 112. As a result, theresistance value of the anode active layer 112 can be reduced and outputof the cell 10 can be enhanced.

Control of the grain size distribution of the 8YSZ powder and the NiOpowder for example is effective to adjust the average value of thecontact length. The average value of the contact length can bemicro-adjusted by the firing conditions or the addition amount andaverage particle diameter of the pore forming agent. In the followingdescription, an example will be given of the preparation conditions forcontrolling the average value of the contact length to a range ofgreater than or equal to 0.4 micrometers and less than or equal to 0.9micrometers as described above.

It is preferred that:

-   -   the average particle diameter of the raw material powder is        greater than or equal to 0.5 micrometers and less than or equal        to 1.3 micrometers, and that microscopic particles having a        particle diameter of less than or equal to 0.2 micrometers are        removed by classification processing,    -   the average particle diameter of the pore forming agent is        greater than or equal to 0.8 micrometers and less than or equal        to 10 micrometers,    -   the addition amount of the pore forming agent is less than or        equal to 20 volume % relative to the ceramic raw material        (8YSZ+NiO), and    -   the co-firing temperature is at least 1350 degrees C. and no        more than 1500 degrees C., and the processing time is at least 1        hour and no more than 20 hours.

The ratio of the number of average contacting spots that have at leastthe average value of the contact length to the total number ofcontacting spots between the Ni particles and the 8YSZ particles(hereinafter referred to as “contacting spot ratio exhibiting at leastthe average value”) is preferably greater than or equal to 25.9% andless than or equal to 48.9%. The distribution graph illustrated in FIG.4 illustrates the contacting spot ratio exhibiting at least the averagevalue corresponding to a ratio of the sum of the frequency of least theaverage value of 0.63 micrometers relative to the total frequency, andtakes a value of 34.0%. The contacting spot ratio exhibiting at leastthe average value is an index that indicates resistance to configurationchange in the Ni particles. That is to say, when the contacting spotratio exhibiting at least the average value is controlled to within apredetermined range, suppression of configuration change of the Niparticles is enhanced, and the durability of the anode active layer 112can be enhanced.

In the same manner as adjustment of the average value of the contactlength, the grain size distribution of the 8YSZ powder and the NiOpowder for example is effective to adjust the contacting spot ratioexhibiting at least the average value. The contacting spot ratioexhibiting at least the average value can also be micro-adjusted by thefiring conditions or the method of mixing the materials. In thefollowing description, an example will be given of the preparationconditions for controlling the contacting spot ratio exhibiting at leastthe average value to a range of greater than or equal to 25.9% and lessthan or equal to 48.9% as described above.

It is preferred that:

-   -   the average particle diameter of the raw material powder (8YSZ,        NiO) is greater than or equal to 0.5 micrometers and less than        or equal to 1.3 micrometers, and that microscopic particles        having a particle diameter of less than or equal to 0.3        micrometers and coarse particles of greater than or equal to 2        micrometers are removed by classification processing,    -   the material forming the anode active layer (printing paste when        formation is performed using a printing method) is uniformly        mixed,    -   when preparing a printing paste as a material for forming the        anode active layer, after preparing a slurry having superior        dispersing characteristics due to addition of a suitable        dispersing agent, the slurry is uniformly mixed by sufficient        application of pot-mill mixing and tri-roll mill mixing, and    -   the firing temperature is at least 1400 degrees C. and no more        than 1450 degrees C., and the processing time is at least 5        hours and no more than 10 hours.

The average value of a first interface length (broken line) between theNi particles and the pores is preferably greater than or equal to 0.2micrometers and less than or equal to 0.7 micrometers. The average valueof the second interface length (broken line) between the YSZ particlesand the pores is preferably greater than or equal to 0.5 micrometers andless than or equal to 1.2 micrometers.

(4) Deviation in Contact Length

In an SEM image with arbitrarily captured 10 fields of view(magnification ×3000), the standard deviation of the average value ofthe contact length of the Ni particles and the 8YSZ particles ispreferably less than or equal to 0.48. The standard deviation of theaverage value of the contact length is an index indicating the bondwidth of the Ni particles and the 8YSZ particles in the anode activelayer 112. That is to say, the uniformity of the conductivity of theinner portion of the anode active layer 112 is enhanced by reducing thestandard deviation of the average value of the contact length, andthereby it is possible to suppress concentration and flow of current ina part of the inner portion of the anode active layer 112. In thismanner, since acceleration of the deterioration of a part of the innerportion of the anode active layer 112 can be suppressed, the durabilityof the cell 10 can be further enhanced.

In the same manner as the adjustment of the contacting spot ratioexhibiting at least the average value, the standard deviation of theaverage value can be adjusted by controlling the average particlediameter of the raw material powder, the mixing conditions of thematerial forming the anode active layer, and the firing conditions.Furthermore, the standard deviation of the average value can be adjustedby controlling the contamination by impurities into the mixing process.By controlling contamination by impurities in this manner, it ispossible to ensure reproducibility and enhance composition controlcharacteristics during co-firing, and to enhance the uniformity of theprogress of sintering of the NiO particles and the 8YSZ particles.Therefore, suppression of variation in the contact length by suppressingcontamination by impurities is particularly effective in order tocontrol the standard deviation of the average value. More specifically,it is sufficient if the contamination by Si, B, Cr, P, S and the like isapproximately less than or equal to 200 ppm.

In an SEM image with arbitrarily captured 10 fields of view(magnification ×3000), the standard deviation of the average value ofthe first interface length of the Ni particles and the pores ispreferably greater than or equal to 0.2 and less than or equal to 0.6.In an SEM image with arbitrarily captured 10 fields of view(magnification ×3000), the standard deviation of the average value ofthe second interface length of the YSZ particles and the pores ispreferably greater than or equal to 0.6 and less than or equal to 1.0.

(5) Ratio of Isolated Ni Particles

FIG. 5 is an FE-SEM image using an in-lens secondary electron detectorand illustrates the analysis results of connected Ni particles andisolated Ni particles.

The use of the in-lens secondary electron detector enables output of thedifference in the conductivity in the respective Ni particles as animage contrast. It is known that particles that have high conductivecharacteristics, that it to say, that have high connectivitycharacteristics in relation to which electrical connectivity with theperipheral Ni particles has been confirmed are displayed in a brightconfiguration, and particles that have low conductive characteristics,that it to say, that have unconfirmed electrical connectivity with theperipheral Ni particles are displayed in a dark configuration (referenceis made to “Solid State Ionics” 178(2008)1984).

FIG. 5 is an FE-SEM image using an in-lens secondary electron detector.In order to confirm that the resulting contrast in the Ni particlesexpresses the difference in the presence or absence of connectivity inthe Ni particles, that is to say, expresses the difference inconductivity, a resistance evaluation of the respective Ni particles wasperformed using an atomic force microscope (AFM) in a single field ofview by scanning a cross section with a cantilever to which apredetermined voltage was applied. At this time, the regions configuredby Ni particles were classified into conductive regions andnon-conductive regions based on the dimension of the current. Theresults of analysis were used to determine that a conductive regionconsists of “connected Ni particles” and a non-conductive regionconsists of “isolated Ni particles”. That is to say, a connected Niparticle is an Ni particle that is connected to at least one adjacent Niparticle, and an isolated Ni particle is an Ni particle that is notconnected to an adjacent Ni particle, and exists in an singularconfiguration. In FIG. 5, the isolated Ni particles are encircled withthe solid line and the connected Ni particles are encircled with thebroken line.

It is preferred that the ratio of the occupied surface area of theisolated Ni particles to the total occupied surface area of the Nipresent in one field of view (hereinafter referred to as “isolated Niparticle ratio”) is less than or equal to 25%. The isolated Ni particleratio is an index of the conductivity of the anode active layer 112.That is to say, as the isolated Ni particle ratio becomes smaller, thereaction resistance of the anode active layer 112 can be decreased, andtherefore, maintenance of the output of the cell 10 can be enhanced.

The isolated Ni particle ratio can be controlled for example byadjusting the powder characteristics (particle diameter, specificsurface area) of the NiO powder that is mixed into the slurry for use inthe anode 11. Furthermore, the adjustment of the isolated Ni particleratio can be controlled by adjusting the firing temperature and thefiring time, or by adjusting the addition amount or the particlediameter of the pore forming agent.

In particular, it is important to use a material with high activity NiOparticles. Use of an NiO material with a high specific surface areaenhances the contacting characteristics with other NiO or 8YSZ duringfiring, and reduces the isolated Ni particle ratio even after a reducingprocess. More specifically, it is preferred to use a material configuredfrom NiO particles that have a specific surface area of 5 m²/g to 20m²/g. When using a material with a high specific surface area, additionof an effective dispersing agent (for example, a wetting dispersingagent such as “DESPERBYK®-180” manufactured by BYK-Chemie Japan) must beadded. This is due to the fact that, in a slurry with low dispersingcharacteristics, particles with a high specific surface area form anaggregate, and conversely inhibit sintering characteristics to therebyimprove the isolated Ni particle ratio. The firing temperature ispreferably as least 1400 degrees C. and less than or equal to 1450degrees C., and the processing time is preferably at least 5 hours, nomore than 10 hours.

When analyzing a SEM image using image analysis software, connected Niparticles and isolated Ni particles can be determined by detaileddetection of contrasts. For example, in the region that is uniformlyclassified as “grey” in FIG. 2, the bright region may be determined tobe “connected Ni particles”, whereas the dark region may be determinedto be “isolated Ni particles”.

In the various methods described above relate to the determination ofconnected Ni particles and isolated Ni particles, the surface area thatis obtained by actually cutting using mechanical processing along thethickness direction (direction of stack) of a sample of the cell 10 isanalysed. Therefore, it may be considered that some of the Ni particlesthat are distributed in proximity to the cross section were connected Niparticles prior to cutting and changed to isolated Ni particles aftercutting (or vice versa). However, it is considered that the ratio of theparticles that changed from connected Ni particles to isolated Niparticles (or vice versa) before and after cutting of the Ni particlesdistributed in proximity to the cross section is extremely small.Therefore, an isolated Ni particle ratio calculated based on the crosssection obtained by cutting a sample of the cell 10 using mechanicalprocessing substantially corresponds with the actual isolated Niparticle ratio prior to cutting a sample of the cell 10.

Method of Manufacturing Fuel Cell 10

Next, an example of a method of manufacture for the cell 10 will bedescribed. Respective conditions such as the material, the particlediameter, the temperature and the method of coating as described belowmay be varied as required. “Green body” below denotes a state prior tofiring. In the following description, as illustrated in FIG. 1, theinterface region R is a portion of the anode active layer 112.

Firstly, a transition metal oxide (for example, NiO powder), and anoxygen ion conductive material (for example, 8YSZ powder), and a poreforming agent (for example, PMMA; Poly(methyl methacrylate resin)) aremixed.

Next, a slurry is formed by adding polyvinyl alcohol (PVA) as a binderto a mixture of the transition metal oxide, the oxygen ion conductivematerial, and the pore forming agent.

Next, the slurry is dried and granulated by use of a spray drier to forma powder for the anode current collecting layer.

Then, the powder for the anode current collecting layer is molded usinga die press molding method to form a green body for the anode currentcollecting layer 111.

Next, the transition metal oxide (for example, NiO powder), and theoxygen ion conductive material (for example, 8YSZ powder), and the poreforming agent (for example, PMMA) are mixed.

Next, a slurry is formed by adding polyvinyl alcohol as a binder to amixture of the transition metal oxide, the oxygen ion conductivematerial, and the pore forming agent.

Next, a first-layer of the green body of the anode active layer 112 isformed by printing the slurry onto the green body of the anode activelayer 111 using a printing method.

Next, the transition metal oxide (for example, NiO powder), and theoxygen ion conductive material (for example, 8YSZ powder), the additive(Si, P, Cr, B and S), and the pore forming agent are mixed.

Next, a slurry is formed by adding polyvinyl alcohol as a binder to amixture of the transition metal oxide, the oxygen ion conductivematerial, the additive, and the pore forming agent.

Next, a second-layer of the green body of the anode active layer 112 isformed by printing the slurry onto the first-layer of the green bodyusing a printing method. With this, the green body of the anode 11 isformed.

When preparing the slurry for the second-layer of the anode active layer112, after firing and reducing, it is preferred that additives are addedso that Si is less than or equal to 200 ppm, the P is less than or equalto 50 ppm, Cr is less than or equal to 100 ppm, B is less than or equalto 100 ppm, and S is less than or equal to 100 ppm. In addition, whenpreparing the slurry for the first-layer of the anode active layer 112,after firing and reducing, it is also preferred that additives are addedin the same manner. The additives are further preferably adjusted sothat respectively in the first-layer and the second-layer, Si is lessthan or equal to 100 ppm, P is less than or equal to 30 ppm, Cr is lessthan or equal to 50 ppm, B is less than or equal to 50 ppm, and S isless than or equal to 30 ppm. It is further preferred to adjust therespective content rate of Si, P, Cr, B and S to be greater than orequal to 1 ppm.

Next, a mixture of 8YSZ powder, water and a binder is mixed in a ballmill for 24 hours to prepare a slurry. Then the slurry is coated on thegreen body for the anode 11 and dried to form a green body for the solidelectrolyte layer 12. In substitution for a method of coating, a methodsuch as a tape stacking method or a printing method may be used.

Then, a mixture of GDC powder, water and a binder is mixed in a ballmill for 24 hours to prepare a slurry. Then the slurry is coated on thegreen body for the solid electrolyte layer 12 and dried to form a greenbody for the barrier layer 13. In substitution for a method of coating,a method such as a tape stacking method or a printing method may beused.

As described above, a stacked body configured from the green body forthe anode 11, the green body for the solid electrolyte layer 12 and thegreen body for the barrier layer 13 can be formed.

Then, the stacked body is co-sintered for 2 to 20 hours at 1300 to 1600degrees C. to form a co-fired body formed from a dense barrier layer 13,the solid electrolyte layer 12 and the anode 11 that includes the anodecurrent collecting layer 111 and the anode active layer 112.

Then, a mixture of water and a binder is mixed with LSCF powder in aball mill for 24 hours to prepare a slurry. The slurry is coated on thebarrier layer 13 of the co-fired body and dried, and fired for one hourin an electric furnace (oxygen-containing atmosphere, 1000 degrees C.)to form the porous cathode 14 on the barrier layer 13. In this manner,the cell 10 is completed.

Operation and Effect

(1) The present inventors conducted diligent research in relation to theproblem that the voltage of the cell 10 is reduced during long-termoperation, and gained the insight that such a phenomenon results from anincrease in the reaction resistance in the interface region R of theanode 11 that is the reaction region between hydrogen and the oxygenions. More specifically, when sintering a molded body for the anode 11,the reduction in the reaction fields for hydrogen and oxygen ions causedby excessive progress of sintering of the transition metal results in anincrease in the reaction resistance in the interface region R.

As a result, the interface region R according to the present embodimentis adjusted so that the content rate of Si is less than or equal to 200ppm, the content rate of P is less than or equal to 50 ppm, the contentrate of Cr is less than or equal to 100 ppm, B is less than or equal to100 ppm, and the content rate of S is less than or equal to 100 ppm.

Therefore, since it is possible to suppress progress of sintering of thetransition metal during sintering of the green body for the anode 11, itis possible to suppress a reduction in the reaction fields betweenhydrogen and oxygen ions. As a result, the increase in the reactionresistance in the interface region R can be suppressed, and a reductionin the voltage of the cell 10 can be suppressed.

Although the mechanism itself by which suppression of the respectivecontent rate of Si, P, Cr, B and S causes a suppression in the progressof sintering of the transition metal is not always clear, it is thoughtto result from the effect of a suitable amount of Si, P, Cr, B and Sbeing taken up into an inner portion of the transition metal particles,or into the grain boundary of the transition metal particles.

(2) The content rate respectively of Si, P, Cr, B and S in the anodeactive layer 112 is greater than or equal to 1 ppm.

Therefore, since Si, P, Cr, B and S function as a sintering auxiliary,suitable sintering characteristics can be maintained in the anode 11(more specifically, the interface region R). In this manner, since thebackbone of the interface region R that has a porous structure can bestrengthened, it is possible to stabilize the porous structure duringfiring and reduction.

Other Embodiments

The present invention is not limited to the above embodiments andvarious modifications for changes are possible within a scope that doesnot depart from the spirit of the invention.

(A) In the above embodiment, although the cell 10 includes the anode 11,the solid electrolyte layer 12, the barrier layer 13 and the cathode 14,the invention is not thereby limited. The cell 10 may include the anode11, the solid electrolyte layer 12 and the cathode 14, and another layermay be interposed between the anode 11 and the solid electrolyte layer12, or between the barrier 13 and the cathode 14. For example, the cell10 may include a porous bather layer between the barrier layer 13 andthe cathode 14.

(B) Although a vertically striped fuel cell has been described in theabove embodiment, the shape of the cell 10 is not limited in thisregard, and the shape of the cell 10 may be a segmented-in-series type,an anode-support type, a tabular-shape type, a cylindrical-shape type,or the like. Furthermore, the sectional surface of the cell 10 may beoval, or the like.

(C) In the present embodiment, the anode active layer 112 is configuredto include at least NiO and/or Ni as a transition metal. However, theinvention is not limited in this regard. The anode active layer 112 mayinclude Fe₂O₃ (or FeO) and/or Fe, CuO and/or Cu or the like, insubstitution for NiO and/or Ni. The anode active layer 112 may alsoinclude a plurality of transition metals (or oxides of the transitionmetal).

(D) In the present embodiment, Si, P, Cr, B and S are mixed with atransition metal and an oxygen ion conductive material. However, theinvention is not limited in this regard. Si, P, Cr, B and S may be mixedwith a binder in a mixture of a transition metal and an oxygen ionconductive material.

(E) In the present embodiment, in particular, the description hasfocused on the characteristic structure of the anode active layer 112.When the anode current collecting layer 111 is composed of the samematerial (for example, Ni-8YSZ) as the anode active layer 112, the anode11 may be configured to have a region in which the average value of thecontact length in proximity to the interface with the solid electrolytelayer 12 is greater than or equal to 0.4 micrometers and less than orequal to 0.9 micrometers. This is due to the fact that it is possible toobtain the desired effect in at least this region. In this region, asdescribed in the above embodiment, the contacting spot ratio exhibitingat least the average value is preferably greater than or equal to 25.9%and more than 48.9%. Furthermore, it is preferred that the standarddeviation of the average value of the contact length of the Ni particlesand the 8YSZ particles is less than or equal to 0.48.

EXAMPLES

The examples of a cell according to the present invention are describedbelow. However the present invention is not limited to the examples asdescribed below.

Preparation of Samples No. 1 to No. 45

As described below, Samples No. 1 to No. 45 of an anode-support typecell were prepared in which the anode current collection layer isconfigured as a support substrate.

Firstly, an anode current collection layer (NiO:8YSZ=50:50 (convertedusing Ni volume %)) having a thickness of 500 micrometers was formedusing a die press molding method.

Next, a slurry was formed by adding polyvinyl alcohol to the mixture ofthe transition metal oxide, the oxygen ion conductive material, andPMMA. The mixing ratio and the type of oxygen ion conductive materialand the transition metal are illustrated in Table 1.

A first-layer green body of the anode active layer having a thickness of10 micrometers was formed on top of the anode current collecting layerusing a printing method.

Next, a slurry was formed by adding polyvinyl alcohol to a mixture ofthe transition metal oxide, the oxygen ion conductive material, anadditive (Si, P, Cr, B and S) and PMMA. The mixing ratio and the type ofoxygen ion conductive material and the transition metal were the same asthose for the green body for the first-layer as described above. Thecontent rate of the additive (Si, P, Cr, B and S) is shown in Table 1.

A second-layer green body of the anode active layer having a thicknessof 10 micrometers was formed on top of the first-layer green body usinga printing method. Next, an 8YSZ electrolyte having a thickness of 5micrometers and a GDC barrier film having a thickness of 5 micrometerswere formed in series on the anode active layer (second-layer) tothereby form a stacked body.

The stacked body was then co-sintered for two hours at 1400 degrees C.to obtain a co-fired body. Thereafter, an anode supporting coin cell(phi=15 mm) was completed by forming an LSCF cathode having a thicknessof 30 micrometers by firing for two hours at 1000 degrees C. Afterfiring, the thickness of the second-layer of the anode active layer(corresponding to the interface region R illustrated in FIG. 1) was 3micrometers.

Measurement of Output Density

Samples No. 1 to No. 45 were heated to 750 degrees C. while supplyingnitrogen gas to the anode and air to the cathode. When 750 degrees C. isreached, a reduction process was performed for three hours whilesupplying hydrogen gas to the anode current collecting layer.Thereafter, the voltage drop rate per 1000 hours for each sample wasmeasured as a deterioration rate. The output density was expressed as avalue at a temperature of 750 degrees C. and a rated current density of0.2 A/cm². In Table 1, a sample having a deterioration rate of more than1.0% is evaluated as x. A sample having a deterioration rate of lessthan or equal to 1.0% is evaluated as ∘. A sample having a deteriorationrate of less than or equal to 0.5% is evaluated as ⊚.

TABLE 1 Standard deviation of Anode active Average value of the averagevalue of Output Anode active layer the contact length the contact lengthdensity Samples layer material composition (%) (μm) (ten fields of view)(mW/cm²) Evaluation No. 46 Ni-SYSZ 35:65 0.36 0.65 465 X No. 47 Ni-SYSZ35:65 0.4  0.58 620 ◯ No. 48 Ni-8YSZ 35:65 0.62 0.54 612 ◯ No. 49Ni-8YSZ 35:65 0.75 0.56 656 ◯ No. 50 Ni-8YSZ 40:60 0.42 0.45 632 ◯ No.51 Ni-8YSZ 40:60 0.53 0.48 645 ◯ No. 52 Ni-8YSZ 40:60 0.63 0.46 666 ◯No. 53 Ni-8YSZ 40:60 0.85 0.43 635 ◯ No. 54 Ni-8YSZ 45:55 0.32 0.45 531X No. 55 Ni-8YSZ 45:55 0.46 0.48 645 ◯ No. 56 Ni-8YSZ 45:55 0.64 0.52672 ◯ No. 57 Ni-8YSZ 45:55 0.78 0.48 664 ◯ No. 58 Ni-8YSZ 45:55 0.920.55 480 X No. 59 Ni-8YSZ 50:50 0.53 0.45 610 ◯ No. 60 Ni-8YSZ 50:500.72 0.53 626 ◯ No. 61 Ni-8YSZ 50:50 0.86 0.49 632 ◯ No. 62 Ni-8YSZ55:45 0.25 0.56 430 X No. 63 Ni-8YSZ 55:45 0.48 0.44 642 ◯ No. 64Ni-8YSZ 55:45 0.75 0.54 656 ◯ No. 65 Ni-8YSZ 55:45 0.90 0.48 620 ◯ No.66 Ni-10YSZ 35:65 0.33 0.35 444 X No. 67 Ni-10YSZ 35:65 0.52 0.55 630 ◯No. 68 Ni-10YSZ 40:60 0.66 0.48 622 ◯ No. 69 Ni-10YSZ 40:60 0.92 0.42425 X No. 70 Ni-10YSZ 50:50 0.65 0.35 630 ◯ No. 71 Ni-10YSZ 50:50 0.720.45 620 ◯ No. 72 Ni—ScSZ 40:60 0.35 0.56 412 X No. 73 Ni—ScSZ 40:600.48 0.50 615 ◯ No. 74 Ni—ScSZ 45:55 0.55 0.45 670 ◯ No. 75 Ni—ScSZ45:55 0.72 0.42 645 ◯ No. 76 Ni—ScSZ 50:50 0.78 0.54 635 ◯ No. 77Ni—ScSZ 50:50 0.30 0.25 412 X No. 78 Ni—Fe-8YSZ 44:1:55 0.33 0.55 428 XNo. 79 Ni—Fe-8YSZ 44:1:55 0.42 0.35 625 ◯ No. 80 Ni—Fe-8YSZ 42:3:55 0.550.48 633 ◯ No. 81 Ni—Fe-8YSZ 41:5:55 0.66 0.62 635 ◯ No. 82 Ni—Fe-8YSZ48:2:50 0.71 0.55 633 ◯ No. 83 Ni—Fe-8YSZ 45:5:50 0.95 0.55 412 X No. 84Ni—Cu-8YSZ 44:1:55 0.36 0.35 422 X No. 85 Ni—Cu-8YSZ 43:2:55 0.55 0.45612 ◯ No. 86 Ni—Cu-8YSZ 42:3:55 0.65 0.52 635 ◯ No. 87 Ni—Cu-8YSZ44:1:55 0.96 0.52 405 X No. 88 Ni-GDC 40:60 0.35 0.25 433 X No. 89Ni-GDC 40:60 0.45 0.52 635 ◯ No. 90 Ni-GDC 45:55 0.65 0.35 655 ◯ No. 91Ni-GDC 45:55 0.75 0.36 666 ◯ No. 92 Ni-GDC 45:55 0.95 0.44 412 X

As illustrated in Table 1, it can be confirmed that the deteriorationrate of the cell (that is to say, the voltage drop rate) can be improvedwhen, in the second-layer (interface region R) of the anode activelayer, the content rate of Si is less than or equal to 200 ppm, thecontent rate of P is less than or equal to 50 ppm, the content rate ofCr is less than or equal to 100 ppm, the content rate of B is less thanor equal to 100 ppm, and the content rate of S is less than or equal to100 ppm.

As illustrated in Table 1, it can be confirmed that the deteriorationrate of the cell (that is to say, the voltage drop rate) can be furtherimproved when, in the second-layer (interface region R) of the anodeactive layer, the content rate of Si is less than or equal to 100 ppm,the content rate of P is less than or equal to 30 ppm, the content rateof Cr is less than or equal to 50 ppm, the content rate of B is lessthan or equal to 50 ppm, and the content rate of S is less than or equalto 30 ppm.

The results are obtained since it is possible to suppress a reduction inthe reaction fields for hydrogen and oxygen ions by suppressing progressof sintering of the transition metal during sintering of the green bodyfor the second-layer of the anode active layer.

In Sample No. 45, it is considered that when the mixed amount of themixture is less than 1 ppm, the strength of the backbone during firingof the anode is insufficient, and sintering progress and causesoccurrence of particle growth of Ni particles after reduction.

Preparation of Samples No. 46 to No. 94

As described below, Samples No. 46 to No. 94 of an anode support cellwere prepared in which the anode current collection layer was configuredas a support substrate.

Firstly, an anode current collection layer (NiO:8YSZ=50:50 (convertedusing Ni volume %)) having a thickness of 500 micrometers was formedusing a die press molding method, and an anode active layer having athickness of 20 micrometers was formed thereon using a printing method.In Samples No. 46 to No. 65, a raw material for the anode active layerwas composed of Ni as the transition metal and 8YSZ as the oxygen ionconductive material. In Samples No. 66 to No. 71, a raw material for theanode active layer was composed of Ni as the transition metal and 10YSZas the oxygen ion conductive material. In Samples No. 72 to No. 77, araw material for the anode active layer was composed of Ni as thetransition metal and ScSZ as the oxygen ion conductive material. InSamples No. 78 to No. 83, a raw material for the anode active layer wascomposed of Ni and Fe as the transition metal and 8YSZ as the oxygen ionconductive material. In Samples No. 84 to No. 87, a raw material for theanode active layer was composed of Ni and Cu as the transition metal and8YSZ as the oxygen ion conductive material. In Samples No. 88 to No. 92,a raw material for the anode active layer was composed of Ni as thetransition metal and GDC as the oxygen ion conductive material. Themixing ratio of the transition metal and the oxygen ion conductivematerial (transition metal:oxygen ion conductive material) was shown inTable 2.

In Samples No. 93 to No. 94, a raw material for the anode active layerwas composed of Ni as the transition metal and 8YSZ as the oxygen ionconductive material. The mixing ratio of the transition metal and theoxygen ion conductive material in Sample No. 93 was 40:60, and in SampleNo. 94, the mixing ratio of the transition metal and the oxygen ionconductive material was 45:55.

The average value of the contact length in the anode active layer, thecontacting spot ratio exhibiting at least the average value, thestandard deviation of the average value of the contact length, and theisolated Ni particle ratio were varied as illustrated in Table 2 andTable 3 by adjusting the powder characteristics (particle diameter,specific surface area) of the NiO powder, the addition amount of thepore forming agent, and the particle distribution of 8YSZ in theformation process of the anode active layer.

Next, an 8YSZ electrolyte having a thickness of 5 micrometers and a GDCbarrier film having a thickness of 5 micrometers were formed in serieson the anode active layer to thereby form a stacked body.

The stacked body was then co-sintered for two hours at 1400 degrees C.to obtain a co-fired body. Thereafter, an anode supporting coin cell(phi=15 mm) for Sample No. 46 to No. 94 was prepared by forming an LSCFcathode having a thickness of 30 micrometers by firing for two hours at1000 degrees C.

Observation of Sectional Surface of Anode Active Layer

The sectional surface of the anode active layer was observed in SamplesNo. 46 to No. 94.

More specifically, firstly, after polishing the anode active layer ofeach sample with precision machinery, ion milling processing wasperformed using an IM4000 manufactured by Hitachi High-TechnologiesCorporation.

Then, an SEM image of the sectional surface of the anode active layerenlarged by 3000 times was obtained by use of an FE-SEM using an in-lenssecondary electron detector (reference is made to FIG. 2).

Then, the contacting line between the Ni particles and the oxygen ionconductive material was examined by analysis of the sectional photographillustrated in FIG. 2 using HALCON image analysis software produced byMVTec GmbH (Germany) (reference is made to FIG. 3).

Next, the analysis results by the image analysis software were used tocalculate the average value of the contact length, the contacting spotratio exhibiting at least the average value and the isolated Ni particleratio with respect to a SEM image for one field of view. Furthermore,the analysis results by the image analysis software were used tocalculate the standard deviation of the average value of the contactlength with respect to a SEM image for ten arbitrary fields of view. Thecalculation results are shown in Table 2 and Table 3.

Measurement of Output Density

Samples No. 46 to No. 94 were heated to 750 degrees C. while supplyingnitrogen gas to the anode and air to the cathode. When 750 degrees C. isreached, a reduction process was performed for three hours whilesupplying hydrogen gas to the anode. Thereafter, the output density ofthe Samples No. 46 to No. 94 was measured. The output density isexpressed as a value at a temperature of 750 degrees C. and a ratedvoltage of 0.8 V.

The measurement results are summarized in Table 2. As illustrated inTable 2, the output density is evaluated as a high output when taking avalue of at least 600 mW/cm². As clearly shown by Table 2, it isdetermined that the average value of the contact length is greater thanor equal to 0.4 micrometers and less than or equal to 0.9 micrometers.This is due to the fact that the bond width of the Ni particles and theoxygen ion conductive material can be sufficiently maintained when theaverage value of the contact length is greater than or equal to 0.4micrometers, and a sufficient electrochemical reaction region can bemaintained after reduction when the average value of the contact lengthis less than or equal to 0.9 micrometers.

The above results are obtained by ensuring a sufficient bond widthbetween the Ni particles and the oxygen ion conductive material and donot depend on the type of oxygen ion conductive material. Furthermore,the same effect can be obtained even when Fe or Cu is included inaddition to Ni as the transition metal.

TABLE 2 Standard deviation of Anode active layer Average value of theaverage value of Output Anode active layer composition the contactlength the contact length density Samples material (%) (μm) (ten fieldsof view) (mW/cm²) Evaluation No. 46 Ni-8YSZ 35:65 0.36 0.65 465 X No. 47Ni-8YSZ 35:65 0.4  0.58 620 ◯ No. 48 Ni-8YSZ 35:65 0.62 0.54 612 ◯ No.49 Ni-8YSZ 35:65 0.75 0.56 656 ◯ No. 50 Ni-8YSZ 40:60 0.42 0.45 632 ◯No. 51 Ni-8YSZ 40:60 0.53 0.48 645 ◯ No. 52 Ni-8YSZ 40:60 0.63 0.46 666◯ No. 53 Ni-8YSZ 40:60 0.85 0.43 635 ◯ No. 54 Ni-8YSZ 45:55 0.32 0.45531 X No. 55 Ni-8YSZ 45:55 0.46 0.48 645 ◯ No. 56 Ni-8YSZ 45:55 0.640.52 672 ◯ No. 57 Ni-8YSZ 45:55 0.78 0.48 664 ◯ No. 58 Ni-8YSZ 45:550.92 0.55 480 X No. 59 Ni-8YSZ 50:50 0.53 0.45 610 ◯ No. 60 Ni-8YSZ50:50 0.72 0.53 626 ◯ No. 61 Ni-8YSZ 50:50 0.86 0.49 632 ◯ No. 62Ni-8YSZ 55:45 0.25 0.56 430 X No. 63 Ni-8YSZ 55:45 0.48 0.44 642 ◯ No.64 Ni-8YSZ 55:45 0.75 0.54 656 ◯ No. 65 Ni-8YSZ 55:45 0.90 0.48 620 ◯No. 66 Ni-10YSZ 35:65 0.33 0.35 444 X No. 67 Ni-10YSZ 35:65 0.52 0.55630 ◯ No. 68 Ni-10YSZ 40:60 0.66 0.48 622 ◯ No. 69 Ni-10YSZ 40:60 0.920.42 425 X No. 70 Ni-10YSZ 50:50 0.65 0.35 630 ◯ No. 71 Ni-10YSZ 50:500.72 0.45 620 ◯ No. 72 Ni—ScSZ 40:60 0.35 0.56 412 X No. 73 Ni—ScSZ40:60 0.48 0.50 615 ◯ No. 74 Ni—ScSZ 45:55 0.55 0.45 670 ◯ No. 75Ni—ScSZ 45:55 0.72 0.42 645 ◯ No. 76 Ni—ScSZ 50:50 0.78 0.54 635 ◯ No.77 Ni—ScSZ 50:50 0.30 0.25 412 X No. 78 Ni—Fe-8YSZ 44:1:55 0.33 0.55 428X No. 79 Ni—Fe-8YSZ 44:1:55 0.42 0.35 625 ◯ No. 80 Ni—Fe-8YSZ 42:3:550.55 0.48 633 ◯ No. 81 Ni—Fe-8YSZ 41:5:55 0.66 0.62 635 ◯ No. 82Ni—Fe-8YSZ 48:2:50 0.71 0.55 633 ◯ No. 83 Ni—Fe-8YSZ 45:5:50 0.95 0.55412 X No. 84 Ni—Cu-8YSZ 44:1:55 0.36 0.35 422 X No. 85 Ni—Cu-8YSZ43:2:55 0.55 0.45 612 ◯ No. 86 Ni—Cu-8YSZ 42:3:55 0.65 0.52 635 ◯ No. 87Ni—Cu-8YSZ 44:1:55 0.96 0.52 405 X No. 88 Ni-GDC 40:60 0.35 0.25 433 XNo. 89 Ni-GDC 40:60 0.45 0.52 635 ◯ No. 90 Ni-GDC 45:55 0.65 0.35 655 ◯No. 91 Ni-GDC 45:55 0.75 0.36 666 ◯ No. 92 Ni-GDC 45:55 0.95 0.44 412 X

Durability Experiment 1

Each of the 12 types samples illustrated in Table 3 was heated to 750degrees C. while supplying nitrogen gas to the anode and air to thecathode. When 750 degrees C. is reached, a reduction process wasperformed for three hours while supplying hydrogen gas to the anodecurrent collecting layer. Thereafter, the voltage drop rate per 1000hours for each sample was measured as a deterioration rate. The outputdensity was expressed as a value at a temperature of 750 degrees C. anda rated current density of 0.35 A/cm².

The measurement results are summarized in Table 3. As illustrated inTable 3, a low deterioration state is evaluated when the deteriorationrate is less than or equal to 1.5%.

As clearly shown in Table 3, it is determined that the contacting spotratio exhibiting at least the average value is preferably greater thanor equal to 25.9% and more than 48.9%. This is due to the fact that aconfiguration change to the Ni particles is inhibited when thecontacting spot ratio is greater than or equal to 25.9%, and asufficient electrochemical reaction region can be maintained to therebyan excessive over-voltage increase can be suppressed when the contactingspot ratio is less than or equal to 48.9%.

As clearly shown in Table 3, it is determined that the isolated Niparticle ratio is preferably less than or equal to 25%. This is due tothe fact that the reaction resistance of the anode active layer can bereduced to thereby facilitate output maintenance when the isolated Niparticle ratio is less than or equal to 25%.

In Sample No. 58 in Table 3, notwithstanding the fact that the isolatedNi particle ratio takes a suitable value of 8.5%, it was not possible toobtain an enhanced effect similar to the other samples since the averagevalue of the contact length exhibited an excessively large value of0.92. This fact shows that the deterioration rate tends to be ratelimited by the average value of the contact length more than by theisolated Ni particle ratio.

As clearly shown by Table 3, it is determined that the standarddeviation of the average value of the contact length is preferably lessthan or equal to 0.48. This is due to the fact that the uniformity ofthe conductivity in the anode active layer is increased and therebyconcentration and flow of current in a part of the anode active layercan be suppressed.

In Sample No. 54 in Table 2, notwithstanding the fact that the standarddeviation of the average value of the contact length takes a suitablevalue of 0.45, it was not possible to obtain an enhanced effect sincethe average value of the contact length exhibits an excessively smallvalue of 0.32. The fact shows that the output density tends to be ratelimited by the average value of the contact length more than by thestandard deviation of the average value.

TABLE 3 Contacting spot ratio Standard deviation of Average value ofexhibiting at least Isolated Ni the average value of Deterioration thecontact length the average value particles ratio the contact length rateSamples (μm) (%) (%) (ten fields of view) (%) Evaluation No. 46 0.3652.1 33 0.65 2.30 X No. 50 0.42 42.3 25 0.45 0.92 ⊚ No. 57 0.78 33.8 80.48 0.32 ⊚ No. 58 0.92 22.2 8.5 0.55 1.45 ◯ No. 65 0.90 25.9 12 0.480.65 ⊚ No. 68 0.66 33.3 4.5 0.48 0.35 ⊚ No. 70 0.65 29.1 9.0 0.35 1.10 ⊚No. 74 0.55 38.9 15 0.45 0.75 ⊚ No. 79 0.42 44.4 10 0.35 0.71 ⊚ No. 910.75 26.5 22 0.36 0.90 ⊚ No. 93 0.46 48.9 18 0.42 0.85 ⊚ No. 94 0.7238.9 6.0 0.33 0.42 ⊚

Durability Experiment 2

In order to examine the relationship between the deterioration rate andthe concentration of the mixture in the anode active layer, Sample No.52 that exhibited a suitable output density was used as a reference tothereby prepare Samples No. 52-1-52-5 by adjusting the mixing amount ofthe mixture as illustrated in Table 4. The mixture is of silicon (Si),phosphorus (P), chrome (Cr), boron (B) and sulfur (S). The mixture maybe premixed into the raw material for the anode active layer, or may bemixed during the preparation process.

In the same manner as the durability experiment 1 described above, eachof Samples No. 52-1-52-5 was heated to 750 degrees C. while supplyingnitrogen gas to the anode and air to the cathode. When 750 degrees C. isreached, a reduction process was performed for three hours whilesupplying hydrogen gas to the anode. Thereafter, the voltage drop rateper 1000 hours for each sample was measured as a deterioration rate. Theoutput density was expressed as a value at a temperature of 750 degreesC. and a rated current density of 0.35 A/cm².

TABLE 4 Additives (ppm) Deterioration Samples Si P Cr B S rate (%)Evaluation 52-1 250 65 125 30 115 1.8 Δ 52-2 200 50 100 100 100 0.88 ◯52-3 135 40 70 80 60 0.76 ◯ 52-4 100 30 50 50 30 0.46 ⊚ 52-5 50 10 20 2010 0.25 ⊚

As illustrated in Table 4, it is confirmed that the deterioration ratecan be improved when the content rate in the entire anode active layerof Si is less than or equal to 200 ppm, of P is less than or equal to 50ppm, of Cr is less than or equal to 100 ppm, of B is less than or equalto 100 ppm, and of S is less than or equal to 100 ppm.

As illustrated in Table 4, it is confirmed that the deterioration ratecan be further improved when the content rate of Si is less than orequal to 100 ppm, of P is less than or equal to 30 ppm, of Cr is lessthan or equal to 50 ppm, of B is less than or equal to 50 ppm, and of Sis less than or equal to 30 ppm.

The above results are obtained due to stabilization of the porousstructure during reduction and during firing due to the mixture of Si,P, Cr, B and S. More specifically, the sintering characteristics of theanode active layer are enhanced by mixing of the mixed materials, andthereby the backbone of the anode active layer that forms the porousbody can be strengthened. On the other hand, control of the mixtureamount of the mixed materials to a micro-level enables suppression ofsintering of Ni during long-term operation, and enables suppression inthe reaction fields in the anode active layer.

The improvement in the deterioration rate due to mixing of respectivelySi, P, Cr, B and S by greater than or equal to 1 ppm was also confirmedexperimentally.

What is claimed is:
 1. A fuel cell comprising: an anode includingtransition metal particles and an oxygen ion conductive material; acathode; and a solid electrolyte layer disposed between the anode andthe cathode; the anode after reduction having an interface region within3 micrometers from the interface between the solid electrolyte layer andthe anode, the anode having an active anode layer with a volume ratio ofNi and NiO that is 25 to 50 volume % and the volume ratio of the oxygenion conductive material that is 50 to 75 volume %, in the interfaceregion, a content rate of silicon being less than or equal to 200 ppmand greater than 1 ppm, a content rate of phosphorus being less than orequal to 50 ppm and greater than 1 ppm, a content rate of chromium beingless than or equal to 100 ppm and greater than 1 ppm, a content rate ofboron being less than or equal to 100 ppm and greater than 1 ppm, and acontent rate of sulfur being less than or equal to 100 ppm and greaterthan 1 ppm, and the content rates of silicon, phosphorus, chromium,boron and sulfur being present in an inner portion of the transitionmetal particles or in a particle interface between the transition metalparticles.
 2. The fuel cell according to claim 1, wherein in theinterface region, the content rate of silicon is less than or equal to100 ppm and greater than 1 ppm, the content rate of phosphorus is lessthan or equal to 30 ppm and greater than 1 ppm, the content rate ofchromium is less than or equal to 50 ppm and greater than 1 ppm, thecontent rate of boron is less than or equal to 50 ppm and greater than 1ppm, and the content rate of sulfur is less than or equal to 30 ppm andgreater than 1 ppm.
 3. The fuel cell according to claim 1, wherein inthe interface region, the content rate of each silicon, phosphorus,chromium, boron and sulfur is greater than or equal to 1 ppm.
 4. Thefuel cell according to claim 1, wherein the transition metal particlescomprise nickel.
 5. The fuel cell according to claim 1, wherein theoxygen ion conductive material is yttria-stabilized zirconia.